This invention relates to a monopulse radar equipment which is applied to, for example, a friend/foe identification apparatus and a secondary surveillance radar equipment for air traffic control.
A monopulse radar equipment has now been developed which is adapted to simultaneously emit radar pulses toward a target object from two antennas of different directionality and to find the direction of the target object from sum and difference patterns of radiation patterns obtained. The apparatus, though being used for the tracking of the target object, has recently been applied to, for example, a secondary surveillance radar equipment and a mode-S system for air traffic control.
In general, the monopulse radar equipment includes first and second radiators arranged in symmetrical positions with respect to a boresight axis of its antenna. The monopulse radar equipment emitting radar pulses symmetrically with respect to the boresight axis through the first and second radiators, extracting horizontal patterns from two directional antenna patterns obtained and entering them into a beam forming network where they are synthesized in an in-phase and opposite-phase fashion to form a sum pattern .SIGMA. and difference pattern .DELTA.. The sum pattern .SIGMA. and difference pattern .DELTA. are entered, through a rotary joint, into a coherent type amplitude comparison multichannel receiver. The multichannel receiver produces, from the sum pattern .SIGMA. and difference pattern .DELTA., an off-boresight signal f (.DELTA./.SIGMA.) corresponding to a deviation of a target object from a boresight axis (where f denotes a function) and a sidelobe suppressed signal log .SIGMA. in which the sidelobe response of the sum pattern is suppressed. These signals f (.DELTA./.SIGMA.) and log .SIGMA. are supplied to a monopulse signal processing circuit to detect the target object and its direction from single radar pulse response.
A brief explanation will be given below about the generation process of the off-boresight signal f (.DELTA./.SIGMA.) and sidelobe suppression signal log .SIGMA..
A .pi./2 hybrid obtains .SIGMA.-j.DELTA. and .DELTA.-j.SIGMA. signals from the sum pattern signal .SIGMA. and difference pattern signal .DELTA.. These signals are converted by mixers to intermediate frequency signals. A phase comparison is made between .pi.-j.DELTA. signal and the reference phase signal -j and between -.DELTA.+j.SIGMA. signal and the reference phase signal -1 both to obtain a cos .phi.. The cos .phi. signals are added together to obtain 2 cos .phi.=f (.DELTA./.SIGMA.), where EQU .phi.=.pi./2-tan.sup.-1 (.DELTA./.SIGMA.)
A logarithmic amplifier receives the .SIGMA. signal and produces a log .SIGMA..
In the monopulse radar equipment using a coherent type amplitude comparison multichannel receiver, it has not been possible to measure a correct direction of the target object if any unbalance occurs between the two channels for the sum .SIGMA. and difference .DELTA.. For this reason, it is required that:
(1) In a feeder between the multichannel receiver and the radiator a balance should be attained in the amplitude (as determined by a feeder loss) and phase (as determined by a feeder length) characteristics between the sum .SIGMA. channel and the difference .DELTA. channel.
(2) In the multichannel receiver a balance should be attained in the amplitude and phase characteristics between the .SIGMA.-j.DELTA. and .DELTA.-j.SIGMA. channels.
Stated in more detail, the following adjustments are required:
(1) An adjustment is made for the symmetrical relation (In the case of an array type antenna the current excitation distribution, and in a reflector-equipped antenna the characteristics and configuration of the radiators and the mechanical strength of the reflector) of the first and second radiators.
(2) An adjustment is made for attaining a balance of the feeder loss and feeder length between the .SIGMA. and .DELTA. channels (including a rotary joint).
(3) An adjustment is made for attaining a balance of the amplitude and phase characteristics between the respective elements and the lines connecting these elements on the .SIGMA.-j.DELTA. and .DELTA.-j.SIGMA. channels.
(4) An adjustment is made for maintaining a predetermined phase relation between the reference oscillation signals which are supplied from a local oscillator to mixers in the conversion of the .DELTA. and .SIGMA. radio frequency signals to the .DELTA. and .SIGMA. intermediate frequency signals and between the reference phase signals -j and -1 which are supplied to phase comparators.
However, these adjustments require a lot of skill and it is difficult to make these adjustments accurately. At the outset, an error budget is properly set to individually adjust the feeder, rotary joint, receiver, etc., but it is difficult to make a final, practical evaluation. Further, since the temperature variation and aging of the individual elements and lines do not occur predictably on both the channels, no proper measure has been made against that situation.